Because
1: This is a party burger. It means it will be cut up into dozens of peices and feed quite a lot of people. Its not just wasted food.
2: If we eat less, there won't be more for them. It doesn't work that way. Our extras get turned into seed and biodegradable waste.
3: If we WERE sending them our extras we would do exactly what we are doing now. Making them dependant on foreign aid, making smugglers and warlords rich while they continue starving.
4: The way to help them, REALLY help them, is to donate to charities that work on building fertilizer plants, watering and water treatment infrastructure and developing drought resistant seed. Give them a chance to become self sufficient and produce enough food to feed themselves. Everything else is just prolonging the hell they live in.

tl;dr complaining that we eat too much or too big is just moralizing ********.